Nervous System Structures and Evolution

• The nervous system controls all other organ systems.
• It is important for maintaining balance within those systems.
• Its role is to send messages to and from the brain and spinal cord, and from the body.
• The ability of cells to respond to the environment evolved over billions of years.

Nervous System Organization

• Nervous systems show diverse patterns of organization.
• Nerve nets connect nerve cords with radial nersves and nerve rings.
• Cephalization results in more complex nervous systems.

Nervous System Divisions

• There are basic divisions of the nervous system.
• The central nervous system (CNS) occupies the cranium and vertebral column.
• The CNS includes the brain and spinal cord.
• The peripheral nervous system (PNS) includes cranial and spinal nerves.

Nervous Tissue

• It consists of two types of cells:
• Neurons are excitable nerve cells which transmit electrical signals.
• Supporting cells are neuroglia, literally "nerve glue."

CNS: Brain Sections

• The CNS (central nervous system) comprises four sections.
• The cerebrum (cerebral hemisphere) makes up a section.
• Diencephalons are sections of the brain.
• The brain stem is a major brain section.
• The cerebellum is a section of the brain.
• The cerebrum is the largest section of the brain.
• The cerebrum has regions for motor movement, sensations, auditory processing, and vision.

PNS Neuroglia

• Schwann cells surround all axons of neurons in the PNS.
• Schwann cells form the myelin sheath around most axons of the PNS.
• Satellite cells surround neuron cell bodies.
• Myelin is a lipoprotein.
• It increases the speed of conduction in large axons.
• Myelin insulates and prevents leakage of electric current.
• Myelin layers have spaces (nodes of Ranvier) between cells.
• Impulses "jump" from node to node.
• "Unmyelinated" axons are smaller and slower.
• In multiple sclerosis (MS), patches of myelin are destroyed in the brain and spinal cord.

CNS Tissue

• Gray matter is gray-colored and is where neuron cell bodies are clustered.
• White matter is white-colored; millions of axons run between different parts of CNS in bundles of "tracts".
• Tracts located specifically in the CNS, versus nerves in PNS.
• White matter comes from the myelin sheaths.

Neuron Structure

• The cell body has a nucleus and cytoplasm with cell bodies clustered.
• CNS clusters are called nuclei.
• PNS clusters are ganglia, and are outside the CNS.
• Dendrites are short and receive nerve impulses for the neuron.
• Axons are long and send nerve impulses away from the cell body.

Neuron Functions

• Neurons can live for a lifetime (over 100 years).
• With few exceptions, neurons cannot divide or replace themselves.
• Neurons have a high metabolic rate.
• They require continuous oxygen and glucose.
• They die within a few minutes without oxygen.

Types of Neurons

• There are three types of neurons based on function and direction.
• Afferent, or sensory nerves carry sensory information from the environment or body to the CNS for interpretation.
• Efferent, or motor nerves carry impulses from the CNS to the PNS to allow for movement or action.
• Interneurons are interpretive neurons between afferent and efferent nerves in the CNS.

Reflexes

• This is a receptor responding to a stimuli
• Sensory neurons carry signals to interneurons
• Interneurons translate the stimulus
• Motor neurons transmit motor output for the stimulus
• Effectors are the "effect" of the stimulus
• Input: Sensory equals sensory input, in which receptors monitor changes.
• These changes are called “stimuli.”
• Information is sent by afferent nerves.
• Integration processes information and decides what should be done.
• Output: Motor equals motor output, in which effector organs (muscles or glands) are activated.
• It is effected by efferent nerves.

Synapses

• These are junctions between neurons.
• Information is passed (usually chemically) and unidirectionally.
• The presynaptic location is toward a synapse and the postsynaptic is away from a synapse.
• Most neurons function as both.
• The synaptic cleft is a tiny gap.
• Neurons can synapse with other neurons, muscle, and glands.
• Information is passed between neurons by chemicals and can be excitatory or inhibitory.
• Along the axons, the information passes electrically.

Central Nervous System Composition

• This system is composed of:
• The spinal cord
• Brain
• Meninges protect the brain and spinal cord.

Spinal Cord

• It is a slender structure continuous with the brain.
• It descends into the vertebral canal and it ends around the level of the first or second lumbar vertebrae.
• The spinal cord carries sensory information to and from the brain.
• It also participates in reflexes, which are unpredictable, automatic responses to stimuli.

Peripheral Nervous System

• This contains:
• Neurons that branch off to the CNS
• Peripheral nerves in two types:
  • Cranial Nerves
  • Spinal Nerves
• Cranial nerves emerge directly from the brain; consists for 12 pairs bring information from the sense organs to the brain; control muscles; connected to glands or internal organs such as the heart and lungs.
• Spinal nerves are peripheral nerves originating from the spinal cord.
• There are 31 pairs of spinal nerves.

Explore the central and peripheral nervous systems and their components. Understand cephalization and the evolution of nervous system complexity. Covers nerve nets, spinal cords, and the ecological advantages of complex neural structures.